1. Field of the Invention
The present invention relates to a device for manufacturing a preform of glass material for optical fibres. More specifically, the invention relates to a device for manufacturing one or more preforms of glass material for optical fibres through a chemical deposition process.
2. Description of the Related Art
As known, the methods for manufacturing optical fibre basically comprise a first process of manufacturing a preform from glass and a successive process of drawing the optical fibre from the preform.
The most common processes of manufacturing preforms comprise one or more chemical deposition steps, through one or more burners, of suitable chemical substances on a cylindrical support; the chemical deposition substances typically comprise silicon and germanium, which are deposited in the form of oxides (SiO2 e GeO2).
The processes of manufacturing preforms through chemical deposition known in the art comprise processes of the VAD (Vapor Axial Deposition) type and processes of the OVD (Outside Vapor Deposition) type.
Typically, in VAD type processes the cylindrical support is held in a vertical position by a gripping member which operates on an upper end of the cylindrical support; the cylindrical support is made to turn upon itself so as to expose its entire surface to one or more burners which are housed near to the lower end of the support and in such a position as to emit a flow of reactants along a direction which is inclined at a predetermined angle, typically lying between 30° and 50°, with respect to the longitudinal axis of the support. The support is then moved upwards so as to allow substantially axial growth of the preform.
In processes of the OVD type, on the other hand, the cylindrical support is held in a horizontal or vertical position by a pair of gripping members which operate on the opposite ends of the support; the support is made to turn upon itself so as to expose its entire surface to one or more burners mounted on a side of the support and in such a position as to emit the flow of reactants along a direction which is substantially perpendicular to the longitudinal axis of the support. The burner, in particular, is mounted on a support structure equipped with a motorised driving member which allows the repeated movement of the burner parallel to the cylindrical support, so as to allow a substantially radial growth of the preform along all the sections of the support.
A typical process of the OVD type comprises the following steps. In a first step a substantially cylindrical glass preform, called “core preform”, is manufactured through deposition of the chemical substances on the cylindrical support: such a preform is named in such a way since it will create the core and a more internal portion of the optical fibre's cladding.
In a second step, the cylindrical support is taken out of the core preform, freeing up a central hole in the preform.
In a third step, the core preform undergoes a process of desiccation and compacting in a furnace, during which suitable gases (comprising, for example, Cl2) are made to flow inside the central hole in order to eliminate the hydroxide ions (—OH) and the atoms of water present in the preform, thus obtaining a vitrified core preform which exhibits a central hole having a smaller diameter than that of the initial preform.
In a fourth step, after having created the vacuum inside the hole, the vitrified core preform is placed in a vertical furnace in which the melting of a lower end of the preform itself is carried out. Such a melting causes the walls of the hole to collapse due to the vacuum created inside of it; the glass material cools down to form an elongated cylindrical element of a predetermined diameter, which is pulled downwards by a suitable traction device. Such an elongated cylindrical element is then cooled down further and cut transversally at many equidistant points so as to form a plurality of elongated elements, also known as “core rods”, typically having a length greater than 1 m and a diameter of between 10 and 20 mm.
In a fifth step, each core rod is used as a substrate for a further chemical deposition process (known as “overcladding”) similar to that of the first step discussed earlier. In particular, on each core rod and through at least one burner, a plurality of chemical substances are deposited (amongst which, typically, there is silicon oxide) which will then constitute the outer portion of the optical fibre's cladding. At the end of the process a low-density final preform is obtained, from which the optical fibre will then be drawn. Before the drawing, the low-density final preform is desiccated and consolidated with the same procedures seen in the third step. In this way a vitrified final preform which is ready for the drawing process is obtained.
Various devices for manufacturing a glass (core or final) preform for optical fibres through processes of the OVD type are known. Such devices typically comprise a chemical deposition chamber inside which are housed the gripping members of the cylindrical support constituting the chemical deposition substrate for the formation of the preform, a burner which is mobile parallel to the longitudinal axis of the cylindrical support, and a suction hood positioned on the opposite side to the burner with respect to the cylindrical support and adapted to collect and remove the particulate and the exhaust chemical substances produced inside the chamber during the chemical deposition.
The chemical substances sucked up by the hoods are then discharged from the chamber through a suitable exhaust pipe and sent to a scrubber.
JP 2000-313625 discloses a device for manufacturing a preform for optical fibres, comprising a plurality of adjacent burners which are mobile parallel to the longitudinal axis of a support for forming a preform, which support rotates upon itself. A suction hood is provided on the opposite side to said plurality of burners with respect to the cylindrical support; said hood also moves parallel to the longitudinal axis of the cylindrical support and in synchrony with said plurality of burners. The hood is associated with a tube for discharging exhaust substances from the chemical deposition chamber through the interposition of a bellows fitting, which allows the hood to be able to move with respect to the tube along the direction parallel to the longitudinal axis of the support. Since the fitting is arranged inside the chemical deposition chamber, it must be realised in a material which, besides having high resistance to mechanical stress (caused by the displacement of the hood), is resistant to high temperatures and acid corrosion which occurs during the chemical deposition.
JP 2001-019463 relates to a technique for manufacturing a porous base material for optical fibers, wherein glass particulates are blown from an oxygen-hydrogen flame burner to an axially rotating rod and are deposited thereon in a reaction vessel, and a moving stage mounted with the burner is moved back and forth between two turning points. An exhaust port is arranged on the side opposite to the burner with respect to the rotating rod and moved in parallel to the burner, to discharge exhaust gases containing unreacted components to the outside of the vessel. The seal between the burner and the reaction container is carried out with a heat-resistant curtain that can be rolled round so that the burner can move. The seal between the exhaust port and the reaction containers is carried out in the same way.
The Applicant has considered the problem of allowing the discharge of the exhaust gases from the chemical deposition chamber equipped with mobile suction elements, looking for solutions which are different from those highlighted above with reference to the state of the art and has identified a simple and at the same time functional solution which provides for the realisation of a coupling of the sliding type between exhaust collector and wall of the chemical deposition chamber.